When creating a future appointment via the backend, the generated lead's activity deadline is incorrectly set to today instead of the appointment date.

### Steps to reproduce

- Install `appointment_crm`.
- Go to Appointments > Schedule > Staff Booking.
- Create a booking for a future date (e.g., next month) and add a customer.
- Confirm the booking.
- Open the newly created Opportunity.
- Check the "Next Activity" deadline.
- It is set to today's date instead of the appointment's date.

### Cause

The `calendar.event` model maintains two sets of fields for timing: `start`/`stop` (Datetime) and `start_date`/`stop_date` (Date). Standard logic dictates that `start_date` and `stop_date` are only populated for All Day events (`allday=True`). For regular time-specific events, these fields are computed as `False` to avoid ambiguity.

When an appointment is created, `appointment_crm` generates a linked CRM Lead and schedules an activity. The code responsible for scheduling this activity (`activity_schedule`) was explicitly passing `event.start_date` as the `date_deadline`.

Since standard backend appointments are time-specific (not all-day), `event.start_date` is `False`. The `activity_schedule` method defaults to the current date (Today) when it receives a falsy value for the deadline. Consequently, creating a future appointment resulted in an immediate deadline.

### Fix

Modify the lead creation logic to use the standard calendar helper `_get_activity_deadline_from_start`.


opw-5780578

Forward-Port-Of: odoo/enterprise#106770
Forward-Port-Of: odoo/enterprise#106162